Top-off charge pump and methods of operating same

ABSTRACT

A charge pump includes a first power source having a voltage V REG  generated from a regulated and circuit-limiter supply, a second power source having a voltage V BRG  and a top-off capacitor adapted to be charged to a voltage of the high of V REG  or V BRG  to a limit of a voltage clamp across the top-off capacitor.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

FIELD

This disclosure relates generally to charge pump circuits, and, moreparticularly, to methods and apparatus for providing a top-off chargepump circuit that also operates with a negative motor battery voltage.

BACKGROUND

As is known, in many electronic circuits, top-off charge pumps are usedin a variety of applications to boost a voltage from an input voltagesource to a higher level to meet system requirements. For example, acharge pump capacitor can be charged to a level to generate a gate drivesignal at a level sufficiently higher than the high side switch sourceterminal in order to turn on the high side switch.

SUMMARY

The present disclosure provides a top-off charge pump circuit andassociated methods wherein a charge pump capacitor is referenced toV_(BRG), which maximizes the drive to the high-side driver in systemswhere V_(BRG) is above V_(BB) by a diode and the top-off charge pump(TOCP) would run off V_(BRG). The circuit protects itself from negativegoing V_(BRG) excursions. The second power supply including an always-oncurrent mirror stops the floating current mirror from turning on and offat the charge pump frequency which is important as it maximizes speed.

In one aspect, a top-off charge pump circuit includes: a first powersource having a voltage V_(REG); a second power source having a voltageV_(BRG); and a top-off capacitor adapted to be charged to a voltage ofthe high of V_(REG) or V_(BRG) to a limit of a voltage clamp across thetop-off capacitor. The charge pump circuit may include one or more ofthe following features either independently or in combination toinclude: wherein the first voltage source comprises a regulated andcircuit-limited supply; wherein the second voltage source comprises analways-on current mirror; wherein a top plate of the top-off capacitoris adapted to provide a voltage of V_(REG) and V_(BRG) up to a limit ofV_(BRG) and V_(CHARGE), where V_(CHARGE) is a clamp voltage across thetop-off capacitor; wherein the top-off capacitor includes a top plateand a bottom plate, the top plate connected to the first power sourcethrough a first diode, the top plate also connected to an output of thecharge pump through a second diode; a first switch having an input andan output, the input connected to the bottom plate of the top-offcapacitor and the output connected to a reference potential; a secondswitch having an input and an output, the input connected to the outputof the second power source and the input connected to the bottom plateof the top-off capacitor; or a diode having an anode connected to theoutput of the second power supply and a cathode connected to the topplate of the top-off capacitor.

In another aspect, a charge pump circuit includes; a first power source,having an output voltage V_(REG); a second power source having an outputvoltage V_(BRG); a top-off capacitor having a top plate and a bottomplate, the top plate connected to the first power source through a firstdiode and adapted to be charged to a voltage the high of V_(REG) orV_(BRG), the top plate also connected to an output of the charge pumpthrough a second diode; a first switch having an input and an output,the input connected to the bottom plate of the top-off capacitor and theoutput connected to a reference potential; a Zener diode disposed inparallel with the top-off capacitor, a second switch having an input andan output, the input coupled to the output of the second power sourceand the input connected to the bottom plate of the top-off capacitor;and a diode having an anode connected to the output of the second powersupply and a cathode connected to the top plate of the top-offcapacitor. The charge pump circuit may include one or more of thefollowing features either independently or in combination to include:wherein the first power source is a regulated and circuit-limitedsupply; wherein the second power source includes an always-on currentmirror; a diode connected in parallel with the first switch; a diodeconnected in parallel with the second switch; wherein a top plate of thetop-off capacitor is adapted to provide a voltage of V_(REG) or V_(BRG).

According to a further aspect, a method of operating a charge pumpincludes: providing a first power source having a voltage V_(REG);providing a second power source having a voltage V_(BRG); charging atop-off capacitor to the voltage level of V_(REG); pumping the top-offcapacitor to increase the voltage level at a top plate of the top-offcapacitor by the voltage V_(BRG); and providing at an output of thecharge pump an output voltage having a voltage V_(REG) and V_(BRG). Withsuch a technique, the top plate of the top-off capacitor is charged to amaximum of a clamp voltage, V_(CLAMP), and then the bottom plate of thetop-off capacitor is connected to V_(BRG) such that the top plate of thetop-off capacitor lifts to V_(BRG)+V_(CLAMP) (max) to provide the outputvoltage. Features of the method may include one or more of thefollowing, either individually or in combination, to include: whereinthe output voltage is at least the higher of V_(REG) or V_(BRG); whereinthe output voltage is limited by a voltage clamp across the top-offcapacitor; or providing protection circuitry to protect the charge pumpwhen the voltage V_(BRG) goes negative.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features may be more fully understood from the followingdescription of the drawing. The drawing aids in explaining andunderstanding the disclosed technology. Since it is often impractical orimpossible to illustrate and describe every possible embodiment, theprovided FIGURE depicts one or more illustrative embodiments.Accordingly, the FIGURE is not intended to limit the scope of the broadconcepts, systems and techniques described herein. Like numbers in theFIGURE denote like elements.

The FIGURE is a circuit diagram depicting an embodiment of a top-offcharge pump according to the disclosure.

DETAILED DESCRIPTION

The features and other details of the disclosure will now be moreparticularly described. It will be understood that any specificembodiments described herein are shown by way of illustration and not aslimitations of the concepts, systems and techniques described herein.The principal features of this disclosure can be employed in variousembodiments without departing from the scope of the concepts sought tobe protected.

In many circuits, a top-off charge pump is required to replenish or“top-up” the charge on a capacitor to maintain a required voltage. Thecapacitor in gate drive systems can be used as a floating chargereference or “battery” to power a high-side driver on a microcircuit.The capacitor's voltage will decay over time if the driver takes staticcurrent, thus a top-off charge pump is used to keep the capacitorcharged up and maintain a high-side on condition indefinitely.

Typically, microcircuits (also referred to as chips or a chip) have adiode from V_(BRG), a motor's battery, to V_(BB) (anode to V_(BRG) side)such that when V_(BRG) goes negative (reverse battery condition), thechip is protected. Reverse battery is usually limited to two diodesbelow ground (the diodes being the reverse body diodes of the MOSFETsthat are being driven). The downside is that this reduces voltageheadroom for circuits running off V_(BB). It is also desirable forcircuits operating in conjunction with a bridge, to run off the bridgesupply voltage. In other systems, V_(BB) and V_(BRG) can be completelydifferent supplies.

This disclosure uses two different power supplies in order to power atop-off charge pump. A first power supply providing a voltage, V_(REG),is generated from a supply voltage V_(BB) typically provided by abattery source. In some instances, the charge pump can be designed tooperate with a battery voltage V_(BB) of between approximately 4.5 voltsand 50 volts. The first power supply is a regulated and current-limitedsupply which is available as long as V_(BB) is available, regardless ofV_(BRG). The other power supply provides a voltage V_(BRG) which can beprovided and derived by a motor battery. As V_(BRG) can potentially gonegative, it is important that the circuit is protected in thiscondition. Furthermore, since V_(BRG) can be a very high voltage, thecircuit needs to be able to handle these voltages and react quickly. Theadvantages of this technique compared to similar techniques include thecharge pump capacitor is referenced to V_(BRG), which maximizes thedrive to the high-side driver in systems where V_(BRG) is above V_(BB)by a diode and the top-off charge pump (TOCP) would run off V_(BB). Thecircuit protects itself from negative going V_(BRG) excursions. Thesecond power supply including an always-on floating current mirrorinstead of a floating current mirror eliminating the need of turning onand off at the charge pump frequency which is important as it maximizesspeed (a large floating epi can take time to charge and discharge).

Referring now to the single FIGURE, charge pump circuit 10 includes afirst power source 20, having an output voltage V_(REG), generated as aregulated and circuit-limited supply and a second power source 30 havingan output voltage V_(BRG). The first power source 20 at an output 24provides a regulated voltage V_(REG) typically in the 8-13 Volt range.The power source 20 can be configured to maintain the regulated voltageat a particular voltage as derived from the voltage of a battery V_(BB)as provided to an input 22. Note a Zener diode 26 is provided as shownto provide a voltage clamp at the input 22. The second power source 30at an output 34 provides a bridge voltage V_(BRG) typically 5 Volts to80 Volts or alternatively a negative voltage and is typically powered atan input 32 by a power source used to supply voltage to a motor orengine. Note a Zener diode 36 is provided as shown to provide a voltageclamp at the input 32. The charge pump circuit 10 further includes atop-off capacitor 40 adapted to be charged to a voltage the high ofV_(REG) or V_(BRG) to a limit of a voltage clamp provided by a Zenerdiode 42 disposed in parallel with the top-off capacitor 40 as shown.The Zener diode 42 typically has a clamping voltage of 20 volts. A topplate 40 a of top-off capacitor 40 is connected to an output 50 of thecharge pump 10 through a diode 54 and connected to the power supply 20through a diode 52 as shown. A bottom plate 40 b of top-off capacitor 40is connected to a switch 2 having a diode 4 disposed in parallel withthe switch 2 as shown. The bottom plate 40 b of top-off capacitor 40 isalso connected to a switch 6 having a diode 8 disposed in parallel withthe switch 6 as shown. The other side of switch 6 is connected to theoutput 34 of power supply 30. A diode 60 has an anode or input connectedto the power supply 30 and a cathode or output connected to the topplate 40 a of the top-off capacitor 40 as shown.

The power supply 30 is provided with a current limiter 38 and an alwayson switch 37 and can be any voltage, for example, 5 Volts to 80 Volts oralternatively a negative voltage and is typically powered by a powersource used to supply voltage to a motor or engine. The power supply 20is provided to maintain here typically 10 volts at the output 50 todrive an external gate and is used to charge the on-chip top-offcapacitor 40. As mentioned above a clamp 42 is also provided in parallelwith the on-chip top-off capacitor 40 to protect the top-off capacitor40.

In operation, when switch 2 is closed or turned on, and the bottom plate40 b is connected to ground and the on-chip top-off capacitor 40 willcharge up to the voltage V_(REG), (minus any incidental voltage dropacross diode 52) for example 10 volts, and the capacitor will maintain10 volts across the capacitor when switch 2 is opened. In the pumpphase, to elevate the capacitor plates of the on-chip capacitor 40,switch 6 is closed and the voltage V_(BRG) is applied to the bottomplate 40 b of the top-off capacitor 40, such that voltage on the topplate 40 a of the capacitor 40 will be V_(BRG) plus V_(REG) (here 10volts in our example) to provide an output voltage at the output 50 ofV_(BRG) plus V_(REG). The output voltage of the charge pump 10 would beV_(BRG) plus V_(REG) and ensures the output voltage is greater thanV_(BRG). It should be noted when V_(BRG) is higher than V_(REG), forexample by 20 or 30 volts, this circuit will charge from V_(BRG) insteadof V_(REG) since travelling through diode 60 will become thepreferential path.

Since V_(BRG) can be attached to a motor battery, V_(BRG) can gonegative, for example −5 volts, so PMOS low-voltage switch 12 isincluded as shown to protect the circuit if V_(BRG) was to go negative.Switch 12 includes a diode 14 disposed in parallel with the switch 12 asshown. The diode has its anode to V_(BRG), so the circuit is protectedfrom negative hits (an external ESD structure can provide an ESD currentpath). For positive V_(BRG) the diode conducts but passive hold-offskeep the circuit turned off until commanded on. When commanded on, theLV PMOS gate is pulled low, turning the PMOS into a switch, and clampsto a Zener diode 36 (cathode on source of PMOS, anode on gate). Itshould be noted it is desirable not to use high voltage devices sincethey are large and expensive, so here a low-voltage device is used witha clamp provided to protect the gate source and a diode across it fromthe other direction to protect it so a so a high voltage switch is notrequired. It should be appreciated, for positive V_(BRG), the diode 14is connected such that an ESD_TOCP node (or “EPI” node labelled in thedrawing) charges up. The switch is turned on by pulling current toground from this node. It clamps on a Zener (not shown) in order toprovide a Vsg voltage to turn the switch 12 on. A resistor (not shown)in parallel with the Zener is a passive pull-off to ensure thegate-source is off with no power applied. When V_(B)RO goes negative,the PMOS switch 12 is off and its body diode blocks.

As described above, in operation of the charge pump 10, in one phase(CHARGE), the top-off capacitor 40 is charged to the higher of V_(REG)or V_(BRG), to a limit of a voltage clamp 42 across the capacitor 40 toa voltage across the capacitor V_(CHARGE). In a second phase (PUMP), thebottom of the capacitor 40 is lifted to V_(BRG), while the top of thecapacitor 40 will be sitting at approximately V_(BRG)+V_(CHARGE). Thisprovides the required voltage at the output 50 with which to supply ahigh-side MOSFET as needed.

The power source 30 includes a floating current mirror 38 implemented inseries with a switch 37, which limits the current out of the powersource 30 in both charge and pump phases. This mirror can uselogic-level devices and the output mirror device uses switch 37 as ahigh-voltage device for protection. The mirror 38 is always on (as longas the TOCP is on) which speeds up operation; a large floating currentmirror can take time to charge and discharge. Switch 37 is closed toactivate the power source 30.

Note that at very high V_(BRG) levels, the capacitor 40 floats relativeto V_(BRG) since the bottom plate 40 b does not go to ground whencharging. The capacitor 40 is charged from V_(BRG) and not V_(REG). Thisis important as we do not want the capacitor 40 to be moving from 0V allthe way to V_(BRG) (which can be as high as 80V) at the clock frequency(a non-overlapping clock determines charge and pump phases). For this towork correctly, the current limited V_(BRG) path current must be greaterthan the current limit which is in line with the charge switch 2. Notethe LV PMOS switch 12 blocks negative V_(BRG) voltages, and thealways-on floating current mirror 38 limits the current at the output 34of the power source 30. It should be appreciated if V_(BRG) is too low,V_(REG) will be high enough to guarantee enough voltage across thetop-off capacitor 40. It should also be appreciated the advantage of thefloating current mirror referenced to V_(BRG) is that it limits thecurrent so that we do not get excess current from the power source 30.

As noted above, the advantages of the described technique compared tosimilar techniques include the charge pump capacitor is referenced toV_(BRG), which maximizes the drive to the high-side driver in systemswhere V_(BRG) is above V_(BB) by a diode and the top-off charge pump(TOCP) would run off V_(BB). The circuit protects itself from negativegoing V_(BRG) excursions. The second power supply including an always-oncurrent mirror stops the floating current mirror from turning on and offat the charge pump frequency which is important as it maximizes speed (alarge floating epi can take time to charge and discharge).

As described above and will be appreciated by one of skill in the art,embodiments of the disclosure herein may be configured as a system,method, or combination thereof. Accordingly, embodiments of the presentdisclosure may be comprised of various means including hardware,software, firmware or any combination thereof. Furthermore, embodimentsof the present disclosure may take the form of a computer programproduct on a computer-readable storage medium having computer readableprogram instructions (e.g., computer software) embodied in the storagemedium. Any suitable non-transitory computer-readable storage medium maybe utilized.

Having described preferred embodiments, it will now become apparent toone of ordinary skill in the art that other embodiments incorporatingtheir concepts may be used. For example, it will also be appreciatedthat while the circuits and techniques are shown and described herein inconnection with analog circuitry, alternatively digital circuitry andtechniques can be used for some or all of the circuit functions.Elements of different embodiments described herein may be combined toform other embodiments not specifically set forth above. Variouselements, which are described in the context of a single embodiment, mayalso be provided separately or in any suitable subcombination. Otherembodiments not specifically described herein are also within the scopeof the following claims. It is felt therefore that these embodimentsshould not be limited to disclosed embodiments, but rather should belimited only by the spirit and scope of the appended claims.

What is claimed is:
 1. A charge pump circuit, comprising: a first powersource having a voltage V_(REG); a second power source having a voltageV_(BRG); and a top-off capacitor adapted to be charged to a voltage ofthe high of V_(REG) or V_(BRG) to a limit of a voltage clamp across thetop-off capacitors; wherein the second voltage source comprises analways-on current mirror.
 2. The charge pump circuit of claim 1comprising a voltage clamp disposed in parallel with the top-offcapacitor.
 3. The charge pump circuit of claim 1 wherein the firstvoltage source comprises a regulated and circuit-limited supply.
 4. Thecharge pump circuit of claim 1 comprising a Zener diode to provide avoltage clamp at an input of the first power supply.
 5. The charge pumpcircuit of claim 1 wherein a top plate of the top-off capacitor isadapted to provide a voltage of V_(REG) and V_(BRG) up to a limit ofV_(BRG) and V_(CHARGE), where V_(CHARGE) is a clamp voltage across thetop-off capacitor.
 6. The charge pump circuit of claim 1 wherein thetop-off capacitor includes a top plate and a bottom plate, the top plateconnected to the first power source through a first diode, the top platealso connected to an output of the charge pump through a second diode.7. The charge pump circuit of claim 6 comprising a first switch havingan input and an output, the input connected to the bottom plate of thetop-off capacitor and the output connected to a reference potential. 8.The charge pump circuit of claim 7 comprising a second switch having aninput and an output, the input connected to the output of the secondpower source and the input connected to the bottom plate of the top-offcapacitor.
 9. The charge pump circuit of claim 8 comprising a diodehaving an anode connected to the output of the second power supply and acathode connected to the top plate of the top-off capacitor.
 10. Acharge pump circuit comprising: a first power source, having an outputvoltage V_(REG); a second power source having an output voltage V_(BRG);a top-off capacitor having a top plate and a bottom plate, the top plateconnected to the first power source through a first diode and adapted tobe charged to a voltage the high of V_(REG) or V_(BRG), the top platealso connected to an output of the charge pump through a second diode; afirst switch having an input and an output, the input connected to thebottom plate of the top-off capacitor and the output connected to areference potential; a Zener diode disposed in parallel with the top-offcapacitor; a second switch having an input and an output, the inputcoupled to the output of the second power source and the input connectedto the bottom plate of the top-off capacitor; and a diode having ananode connected to the output of the second power supply and a cathodeconnected to the top plate of the top-off capacitor; wherein the secondvoltage source comprises an always-on current mirror.
 11. The chargepump circuit of claim 10 wherein the first power source is a regulatedand circuit-limited supply.
 12. The charge pump circuit of claim 10comprising a Zener diode to provide a voltage clamp at an input of thefirst power supply.
 13. The charge pump circuit of claim 10 comprising adiode connected in parallel with the first switch.
 14. The charge pumpcircuit of claim 10 comprising a diode connected in parallel with thesecond switch.
 15. The charge pump circuit of claim 10 wherein a topplate of the top-off capacitor is adapted to provide a voltage ofV_(REG) and V_(BRG).
 16. A method of operating a charge pump comprising:providing a first power source having a voltage V_(REG); providing asecond power source having a voltage V_(BRG), the second voltage sourcecomprising an always-on current mirror; charging a top-off capacitor tothe voltage level of V_(REG); pumping the top-off capacitor to increasethe voltage level at a top plate of the top-off capacitor by the voltageV_(BRG); and providing at an output of the charge pump an output voltagehaving a voltage V_(REG) and V_(BRG).
 17. The method of claim 16 whereinthe output voltage is at least the higher of V_(REG) or V_(BRG).
 18. Themethod of claim 17 wherein the output voltage is limited by a voltageclamp across the top-off capacitor.
 19. The method of claim 16comprising providing protection circuitry to protect the charge pumpwhen the voltage V_(BRG) goes negative.
 20. The method of claim 16comprising providing a Zener diode to provide a voltage clamp at aninput of the first power supply.